mGCA: Validate const literal against expected type

[reddevilmidzy]

close: #151625
close: #150983

[BoxyUwU]

I think in most cases we actually don't want to emit errors during HIR ty lowering for type mismatches here. E.g. if we have a true literal it doesn't really matter if the expected type is u32, we can still lower to a 0x1 valtree of type bool and then get a type checking error later.

Separately from this, we already do these checks in fn lit_to_const (just ICEing if they don't hold instead of a proper error) so it probably doesn't make sense to duplicate them here too 🤔

I think it would make sense for lit_to_const to return an Option<ty::Value<'tcx>> and stop ICEing/erroring altogether.

You could then:

• Move const_lit_matches_ty to THIR building for patterns (the only place other than const-generics that calls lit_to_const) and create a ty::ConstKind::Error if it's false (or if lit_to_const returns None)
• Here in HIR ty lowering we can emit a proper error about needing more type information for the literal if lit_to_const returns None. And we can entirely ignore the possibility of type mismatches because type checking should take care of that for us

[BoxyUwU]

very cool :) thanks for working on this